Silicon is an essential nutrient for plants and is present as low concentrated orthosilicic acid (H2SiO4) in soil, minerals and ocean water. In modern agriculture systems, the nutrient solutions are mostly deficient in orthosilicic acid and the added silicates are unable to compensate for this deficiency. Silicic acid is sometimes included in formulations of nutrients but is not enough bioavailable as such, because they are as silicates poorly soluble in water.
Silicates are not well absorbed by organisms. Probably, orthosilicic acid is the highest bioavailable silicon compound for diatomes, plants, animals and humans. In water, silicates and silicagel are slowly hydrolysed into orthosilicic acid, which is poorly soluble and polymerises quickly into small particles (non-colloidal material (non-opalescent, non-turbid)). These polymerised structures directly aggregate into longer chains (still non-colloidal), leading to a real network (colloid; opalescent, turbid). This process results in the formation of a soft gel, which is poorly bioavailable. The formation of these colloids and gels is pH dependent. The longest gelling time occurs at pH 2. At lower and more alkaline pH, the time for colloid and finally gel formation decreases (Iler R. K. The Chemistry of Silica. Wiley: New York, 1979). According to this reference, the stages from monomer to sol-gel polymerization can be summarised as follows:                1. monomer orthosilicic acid in acid medium;        2. polymerisation of orthosilicic acid monomers into small oligomers (mainly dimers, trimers and tetramers, linear or cyclic);        3. further condensation into linear or randomly branched polymers (small particles, ±2 nm) (pre-sol);        4. growth of these particles (sol, colloidal, particle size of about 5-100 nm);        5. linking of particles into chains (aggregation, colloidal);        6. chained into network and extension throughout the liquid (aggregation, pre-gel);        7. thickening into a gel (gel).        
According to the literature, silicon helps in hardening the roots of plants, and is also essential for good plant growth and disease resistance. Leafs are strengthened through silicic acid formation which acts as a mechanical barrier. Silicon also connects plant substances such as sugars, proteins or phenolic compounds which are present in all kinds of plant fibers. Mycelia of fungi cannot penetrate the plant anymore. It increases the yield, induces resistance to stress, controls diseases and pests, reduces toxicity of certain minerals as manganese and aluminium, increases tolerance to freeze calamities, regulates water consumption and improves leaf erectness, resulting in photosynthesis enhancement. It is described that silicon is absorbed via the roots as orthosilicic acid. Usually, silicates, silica gel (kieselgel), meta-silicates, zeolites and other silicon compounds are used, however, having a low bioavailability.
New chemicals that are used in agriculture also induce polymerisation and aggregation of orthosilicic acid into colloids (e.g. fluorides, nitro- and chlorinated compounds, insecticides, antibiotics, fungicides etc.). By that, synergetic activity between roots and microbes, resulting in better bioavailability of minerals and solubilisation of silicates is omitted or reduced, which results in weaker plants with a lower mineral content. To circumvent this problem, plants have to get more fertilizers than necessary and also have to be protected by insecticides, fungicides, etc. more than necessary. This is especially a problem for plants on hydroculture.
In addition to the importance of silicon to plants, there is also evidence that silicon is an essential element for animals and humans (DE19530882). The question arises if silicon is also able to protect and strengthen animals and humans against infiltration of pathogenic microbes (bacteria, fungi) and could directly be related with certain physiological conditions. The human body contains a very substantial amount of silicon, far higher than most essential trace elements like Mn, Fe, Cu or Zn. Especially organs, connective tissue, cartilage and bones contain high amounts of silicon. Some studies show that the silicon contents decrease with age. Pregnant women have low silicon serum concentrations and the use of silicon supplements by them showed therapeutic action on the skin and lowers aluminium toxicity (Reffitt D. M, Jugdaohsingh R, Thompson R. P. H, Powell J. J.: Silicic acid: its gastrointestinal uptake and urinary excretion in man and effects on aluminium excretion. J. Inorg Biochem 1999; 76:141-6; and: Van Dyck K., Van Cauwenbergh R., Robberecht H., Deelstra H.: Bioavailability of silicon from food and food supplements. Fresenius J. Anal. Chem. 199; 363: 541-4). The use of silicon supplements also reduces aluminium toxicity. Aluminium inhibits bone formation and is correlated with neurological diseases like Parkinson and Alzheimer. Silicon is connected with the elasticity of the artery and blood vessel walls and enhances the immune system.
There are clinical reports on improvement of skin diseases, heart diseases, asthma, rheumatic diseases, psoriasis, bone diseases, etc. by using silica gels. Silica gels are used all over the world. However, these gels are poorly bioavailable because of difficulties to dissolve colloidal silicic acid.
Hence, to use silicon in an effective bioavailable way, one has to use a non-colloidal orthosilicic acid solution and one has to prevent colloid and gel formation. However, it is very difficult to inhibit colloid and gel formation in highly (>10−4 mol Si) concentrated solutions at all pH values. Colloids and gels are not bioavailable but the colloids depolymerise slowly into smaller particles and orthosilicic acid. This depolymerisation is limited and not very reproducible since these colloids are relatively unstable and the polymerisation depends on water content, pH and salt concentration. This results in a very low concentration of orthosilicic acid, which sticks onto all kind of biological materials, in gastro-intestinal systems and rest colloidal material.
Next to silicon, boron is also considered as important trace element. Boron is a well documented essential element for plants. Deficiency results in growth inhibition (Ishii T, Matsunanga T, Hayashi N. Formation of rhamnogalacturonan II-borate dimer in pectin determines cell wall thickness of pumpkin tissue. In: Plant Physiology; 126: (4) 1698-1705 August 2001), and boric acid delays senescence of cornation flowers (Serrano M, Amoros A, Pretel M. T, Martinez-Madrid M. C, Romojaro F. Preservative solutions containing boric acid delay senescence of carnation flowers. Postharvest Biology and Technology, 23: (2) 133-142; November 2001). High concentrations of boron in water gives decreased crop yields. Boric acid is used as fungicide, insecticide and herbicide at different but high concentrations. As herbicide, it is a strong poison. It can act as desiccation compound or it can inhibit photosynthesis and suppress algae in swimming pools and sewage systems. As fungicide, it is used as a wood preservative. Boric acid is therefore used in agriculture and non-agriculture sites, especially in food and feed handling areas.
Boron is also used in humans for healing wounds, vaginal infections, in eye washes, in cosmetics, and in food as preservative or antimicrobial compound, as mild antiseptic. It also should have antiviral activity. The high toxicity limits its use as antimicrobial compound in animals and humans. Before 1980, boron was considered as a non-essential element in human nutrition. Recently numerous animal and human studies showed that it is also essential for normal growth as it is for plants and it is important for hormones involved and bone metabolism (testosteron and estrogen). It is also involved in bone mineralisation.
In nature boron (like silicon) is found in volcanic and other natural water (mineral springs) sources, and also as borates in minerals.
Combinations of silicon and boron in food additives or as medicaments are known from the literature. In e.g. DE19530882 a medicament is used that comprises 21.43 wt. % silicon (from silicea) and 2.14 wt. % boron (from borax). This medicament is used as solid or as liquid. A clear disadvantage is that silicon is not bioavailable in this way. Another document WO 00/27221 describes a solution to concentrate metals in plants, comprising at least 100 mg/kg silicon and at least 100 mg/kg boron. Here it is also a disadvantage that silicon is not, or hardly bioavailable. Also the ranges in which silicon and boron can be added, may lead to combinations which can have a negative effect on the bioavailability. For example, in humans high silicon intake may result in lithiasis, immunological effects or silicon accumulation. Both elements interfere also with the absorption of other minerals. High boron intake may increase testosteron and estrogen levels and may interfere with the parathyroid hormone function.
Boric and silicic acid are weak acids and poorly soluble in water. They are common in non-polluted water all over the earth and vital for mineral balance of plants, animals and humans. All these acids become depleted in polluted systems and their bioavailability decreases.
Also other combinations found in the literature do not use silicon in its bioavailable form and do not use the synergetic effect of boron on the bioavailability of non-colloidal silica. Further, there is also a need for a solution with high concentration of silicic acid, that can be used as stock solution, in which silicic acid is present in its non-colloidal form, notwithstanding its high concentrations and the presence of boron.
It is the object of the invention to make a solution with an increased bioavailability and activity of silicon (in the form of silicic acid) in the presence of boron (in the form of boric acid) in that solution. It is another object of the invention to prepare a high concentrated solution of silicic acid that does not polymerise and/or gel, that can be kept as stock solution for a long period, without polymerization of gelling of that solution in combination with boric acid.